Tree-felling machines are used in mechanized tree harvesting for cutting down trees instead of using a manual chain saw. Each machine comprises a logging vehicle with a boom at the end of which is provided a tree-felling implement. In use, to cut down an adjacent tree, the operator moves the implement towards the base of the tree. The implement then grabs a trunk with a set of hydraulically actuated arms and cuts the trunk at the base with a saw or a felling shear. The trunk is laid down on the ground or in a truck afterwards. Some implements can also remove branches from the trunk while still grabbing it, otherwise branches may be removed by another machine or a manual chain saw, or even not be removed at all.
Because the boom is generally moving only in the vertical plane, the implement has to be able to rotate with reference to the boom plane so that it may grab an inclined or curved tree, or a tree laying on the ground. In conventional tree-felling machines, the implement is rotated by means of a single oblique hydraulic actuator having one end operatively attached to the boom and another end operatively attached to the implement. With such embodiment, the implement may not be rotated around its pivot according to a wide range angle and this alters greatly the flexibility of the machine. In some situations, the operator may have to move the machine around the tree until a suitable angle is found, which may not be always possible because of the usually soft soil and all the obstacles in a logging site. This therefore lowers productivity and adds unnecessary wear to the machine parts.